1. Field of the Invention
The present invention relates to a switch having a first and at least a second external terminal as well as a temperature-dependent switching mechanism that creates, as a function of its temperature, an electrically conductive connection between the two external terminals for an electrical current to be conducted through the switch, the switching mechanism comprising a switching member which changes its geometrical shape between a closed position and an open position as a function of temperature and, in its closed position, carries the current flowing through the switch, as well as an actuating member which is permanently connected electrically and mechanically in series with the switching member.
2. Related Prior Art
A switch of this kind is known from U.S. Pat No. 4,636,766.
The known switch comprises, as the switching member, a U-shaped bimetallic element having two legs of different lengths. Attached to the long leg is a movable contact element which coacts with a switch-mounted countercontact that in turn is connected in electrically conductive fashion to one of the two external terminals.
The shorter leg of the U-shaped bimetallic element is attached to the free end of an actuating member configured as a lever arm, which at its other end is joined immovably to the housing and is connected in electrically conductive fashion to the other of the two external terminals. The actuating member is a further bimetallic element which is matched to the U-shaped bimetallic element in such a way that when temperature changes occur, the two bimetallic elements deform in opposite directions and thus maintain the contact pressure between the movable contact element and the housing-mounted countercontact.
This switch is intended as an interrupter for high currents, which cause considerable heating of the bimetallic element through which current is passing, thus ultimately lifting the movable contact element away from the fixed countercontact. Ambient temperature influences are compensated for, in this context, by the aforementioned opposite-direction deformation of the bimetallic elements.
The principal disadvantage of this design is that two bimetallic elements are required, the temperature characteristics of which must be exactly matched to one another; this is physically complex and cost-intensive to implement. In order to compensate for production tolerances, the known switch is moreover mechanically adjusted after assembly, constituting a further disadvantage.
Since the two bimetallic elements are of geometrically very different design, they also have different long-term stabilities, so that readjustment would in fact be necessary from time to time. This is, however, no longer possible during use, so that long-term stability and thus functional reliability generally leave much to be desired.
A further disadvantage of this design consists in the large overall height resulting from the U-shaped bimetallic element.
The known current-dependent switch is thus of complex design, expensive, and not very reliable.
A further current-dependent switch known from EP 0 103 792 B1 has as the switching member a bimetallic spring tongue which is attached to the one external terminal and at its free end carries a movable contact element which coacts with a countercontact that is arranged at the free end of an elongated spring element that is attached at the other end to the other external terminal. The switch is connected with its external terminals in series with an electrical device in such a way that the operating current of that switch flows through the bimetallic spring tongue. As a rule, the known switch is moreover thermally coupled to the electrical device, so that it can follow its temperature changes.
If the temperature of the device now rises above an impermissible value, the bimetallic spring tongue lifts the movable contact away from the countercontact, thus interrupting the flow of current and preventing the electrical device from heating up further. The bimetallic spring tongue can also, however, be brought into this open position by an increased flow of current, since the bimetallic spring tongue heats up due to the electrical current flowing through it. The electrical properties of the bimetallic spring tongue can be set, in coordination with the mechanical properties and the kickover temperature, in such a way that it is in its closed position, in which it conducts the operating current of the electrical device, when the ambient temperature is below the switching temperature and the operating current is also below a response current intensity. If the operating current then rises above the permissible value, the bimetallic spring tongue heats up very rapidly and reaches its kickover temperature, whereupon it transitions into its open position.
This switch thus offers protection from both overtemperature and overcurrent.
Because of the elastic mounting of the countercontact, the contact and countercontact rub against one another during switching operations, so that contaminants and deposits are rubbed off the contact surfaces, ensuring a low contact resistance and thus a good electrical connection. The elastic mounting of the countercontact furthermore ensures low mechanical loading of the bimetallic spring tongue, since the countercontact gives away to a limited extent. This prevents irreversible deformations of the bimetallic spring tongue. Since mechanical deformations of this kind can lead to a shift in the switching temperature, the overall result of this arrangement is to ensure high operating reliability.
A disadvantage with this known switch, however, is that because of the elastic deflection of the countercontact and the kickover of the bimetallic spring tongue into the open position, it requires a relatively large amount of space for the switching function of the temperature-dependent switching mechanism. A further disadvantage is the fact that during the transition from the closed position into the open position or vice-versa, the bimetallic spring tongue--like all bimetallic elements passes through a so-called "creep" phase in which the bimetallic element deforms in creeping fashion as a result of a rise or drop in temperature, but does not snap over from its, for example, convex low-temperature position directly into its concave high-temperature position. This creep phase occurs each time the temperature of the bimetallic element approaches the kickover temperature from either above or below, and leads to appreciable changes in conformation. The creep characteristics of a bimetallic element can moreover also change even further as a result, particularly, of aging or long-term operation.
During the opening movement, creep can cause the pressure of the contact against the countercontact to weaken, thus leading to undefined switching states. During the closing movement, the contact can gradually approach the countercontact during the creep phase, thus possibly creating the risk of arcing.
In a bimetallic switch known from U.S. Pat No. 4,389,630, this creep phase of the bimetallic element is suppressed by the fact that the bimetallic disk used therein is pressed down in the central region or held down at the rim, buttresses being provided which help to effect the snapover operation. In this context, the contact pressure rises continuously with increasing temperature until opening occurs.
This is achieved by the fact that the bimetallic disk is attached at the free end of a spring element, the joining point between spring element and bimetallic disk being reinforced by a housing-mounted lug. The bimetallic disk is thus placed under mechanical preload, which suppresses the creep phase.
This design is on the one hand complex, a further disadvantage consisting in the fact that the preloading of the bimetallic disk disadvantageously impairs service life and the reproducibility and long-term stability of the switching temperature. If the bimetallic disk were nevertheless to have a greater creep phase, this would impair the function of the switch.
These problems associated with the creep behavior of a bimetallic element are solved, in the case of a current-dependent switch as described in the aforementioned U.S. Pat. No. 4,636,766, in U.S. Pat. No. 4,389,630, or in EP 0 103 792, by the fact that the bimetallic spring tongue is equipped with dimples which do not suppress the creep phase completely, but do suppress it for the most part. These dimples or other actions upon the bimetallic element are complex and expensive features which moreover greatly reduce the service life of these bimetallic elements. A further disadvantage of the requisite dimple may be seen in the fact that not only different material compositions and thicknesses, but also different dimples, must be used for various performance classes and response temperatures.
On the whole, therefore, the disadvantages with these switches are not only the space requirement for the switching operation itself but principally the complex and thus expensive switching member, which moreover must be individually designed in each case for different switch types.
A further design having a movable countercontact is also shown by U.S. Pat. No. 4,319,214. The bimetallic switching mechanism comprises a co-moving countercontact mounted on a spring arm, as well as a movable contact element mounted on a bimetallic arm. The bimetallic arm either is attached directly to the lower housing part or is carried by a further bimetallic arm which in turn is attached to the lower housing part. In either case, the bimetallic arm is equipped with a dished portion for setting the defined snapover point, a counterbearing about which the corresponding bimetallic arm pivots in the event of temperature changes being associated either with the bimetallic arm or with the further bimetallic arm.
In the embodiment having the two bimetallic arms, the latter are matched to one another in terms of their switching behavior and bend about the counterbearing, if applicable, when the temperature rises, the movable contact element being moved away from the countercontact which nevertheless is readjusted due to the spring effect of the spring arm. When the switching temperature is reached, the bimetallic arm snaps over around the dished portion and, if applicable, the counterbearing, thus lifting the movable contact element away from the countercontact, which is prevented by a stop from following the contact element even farther.
These two designs are disadvantageous on the one hand because the creep phase of the bimetallic arm which is permitted only to a limited extent must be matched exactly to the spring force of the spring arm and to the physical position of the stop, so as to prevent the spring arm from reaching the stop prematurely during the creep phase, which would lead to undesired opening of the contacts before the kickover temperature is reached.
To prevent this, the bimetallic arms must be additionally equipped with dished portions limiting the creep phase, and moreover are braced approximately centeredly against a counterbearing about which they bend correspondingly.
Because of these features, the bimetallic arms are exposed to severe mechanical loads which here again have a disadvantageous effect on service life and on the reproducibility and stability of the switching temperature. Because the tolerances are permitted only within very tight limits, this switch is moreover complex and expensive.
A further disadvantage consists in the moving countercontact, which is not only complex in terms of design but here again undesirably increases the overall height because of the travel required.
A further disadvantage with the embodiment having the two bimetallic arms is that, like the generic switch mentioned at the outset, they must be exactly matched to one another in terms of their temperature characteristics in order to define the switching temperature.
In all the switches from the prior art described so far, the creep phase is thus kept as short as possible, increasing or compensating pressure as well as additional dished portions being used for the purpose.
In this connection, DE 21 21 802 C discloses a further temperature-dependent switch in which the switching mechanism comprises a spring disk which, when the switch is in the closed state, is braced with its rim on a first connection electrode and presses a centrally carried movable contact against a stationary countercontact which is provided on a second connection electrode. In the known switch, the two connection electrodes constitute an encapsulated metal housing, and are electrically insulated from one another by an insulating disk.
A bimetallic snap disk which, below its switching temperature, lies loosely in the interior of the known switch (i.e. is not exposed to any mechanical stresses) is slipped over the movable contact. In this switch, the operating current of the device being protected flows only through the spring disk; the bimetallic snap disk is not acted upon by the operating current.
With this switch, the creep phase of the bimetallic snap disk has very much less of an effect than with the switches mentioned previously, so that relatively economical switching members, which moreover have a long service life, can be used here.
When the bimetallic snap disk is heated above its switching temperature, at the end of the creep phase it suddenly jumps from its convex shape into a concave shape, and in the process braces with its rim against the cover of the housing and, with its center region, presses the movable contact away from the countercontact against the force of the spring disk, thus interrupting the circuit.
To ensure that a current cannot now flow via the bimetallic snap disk to the spring disk, an additional insulating disk is provided between the bimetallic snap disk and the cover of the housing, preventing this undesired current flow.
Although this switch is extremely reliable in technical terms and has had great economic success, it is still of too complex a design for certain applications. For example, a specially matched spring disk must always be used as a function of the switching temperature, convexity, and thickness of the bimetallic snap disk; this is altogether complex and expensive. A further disadvantage may be seen in the additional insulation between the bimetallic snap disk and the cover of the switch.
A further disadvantage for certain applications lies in the fact that this switch is not current-dependent, since the bimetallic snap disk does not at any point carry the operating current. It is now commonly known, however, to equip the switch with a series resistor through which the operating current flows and which, if the current flow is too high, heats up correspondingly and causes the bimetallic snap disk to kick over. These design variants are also technically very reliable, but as compared with the switches mentioned at the outset they have the disadvantage that the series resistor cannot react as quickly and sensitively as the bimetallic element of the switch, through which the current flows.